Epoxy compositions are well known in the industry for a variety of uses including structural adhesives. The compositions are typically classified as either one-part, in which the catalyst or curing agent is admixed with the rest of the ingredients, or two-part, in which the catalyst is kept separate from the epoxy composition and mixed only when the epoxy composition is to be used.
In certain applications it is desirable to increase the speed at which the epoxy composition reacts so that a "green strength" can be achieved when the composition is heated for a very short period of time. As used herein, the term "green strength" refers to the shear strength of an epoxy composition that has been partially cured, i.e., some, but not all of the epoxy groups are reacted. Upon further heating, the epoxy composition cures and reaches its ultimate strength wherein most of the epoxy groups are reacted. An example of such an application is in the automotive industry where steel surfaces are bonded together with epoxy adhesives by induction heating. The adhesive is fully cured by thermal heating at a subsequent time.
There have been various attempts to make compositions having the optimum balance of properties between structural adhesive strength, speed of cure, and pot life. However, it has been found that as the speed at which the curing reaction occurs increases, the pot life decreases for one-part compositions. Additionally, achieving complete reaction, i.e., wherein substantially all of the epoxide functional groups are consumed, may require longer cure times and/or higher temperatures.
Corley (U.S. Pat. No. 4,668,758) discloses an epoxy resin composition containing a cationic curing agent and a cure-retarding amount of a diarylamine.
Robins (U.S. Pat. No. 4,503,211) discloses the use of an epoxy resin curing agent comprising the liquid salt of a substituted pentafluoroantimonic acid and an aromatic amine selected from the group consisting of aniline and hindered amines including substituted anilines having an electron-donating substituent on the benzene ring.
Tarbutton et al. (U.S. Pat. No. 4,846,905) discloses thermally curable one-part epoxy compositions catalyzed by a liquid salt of a substituted pentafluoroantimonic acid and N,N-diethyl-o-toluidine.
Buck et al. (U.K. Pat. Spec. No. 963,058) discloses room temperature storable epoxy resin compositions with a curing agent provided by the amine salt of hydrofluoroboric acid.
For applications such as induction heating to bond steel surfaces, and especially to oily steel surfaces, it would be desirable to have an epoxy resin composition which has an extended pot life after mixing, but which will also develop green strength rapidly upon application of heat over a broad temperature range of about 50.degree. C. to about 200.degree. C. The composition should also provide good structural adhesive properties for bonding as measured by lap shear and T-peel.